Since the present invention is most beneficial when used in conjunction with an electronic reclosure control system, specific background information regarding reclosers and electronic reclosure control systems is set forth hereinafter. A recloser is defined by the IEEE Guide for Protective Relaying of Utility-Consumer Interconnections, ANSI/IEEE Std. C37.95-1989, as a device which automatically closes a circuit-interrupting device following an automatic tripping. The reclosing is initiated by an electronic reclosure control system, and may be programmed for any combination of instantaneous, time-delay, single shot, multiple-shot, synchronism-check, dead-line-live-bus, or dead-bus-live-line operation.
Reclosers are inserted into power lines to protect a power distribution system. Most faults on power distribution lines are of the momentary nature and of sufficient magnitude to blow fuses if the fault is allowed to be conducted to the fuse. When a fuse does blow in a power distribution system, it is necessary to dispatch an electrician to replace the blown fuse, which can be an expensive proposition. Furthermore, a blown fuse will result in a substantial period of downtime for a process, thereby resulting in a substantial loss of profit.
A primary function of a recloser is to prevent momentary faults from being conducted to fuses. In general, this goal is achieved by having the reclosure control system sense the value of the current conducted in the power line and interrupt its flow by opening or tripping a recloser before the fuses can blow. After an interval, the reclosure control system allows the recloser to close, thereby restoring power to the system. The recloser remains closed until the next fault is sensed. The rate at which a fuse will blow, and thus interrupt current, is generally a function of the thermal heating of the fusible element. The rate of thermal heating is proportional to the current consumed by the fault. Each fuse generally has a logarithmic time vs. current characteristic curve which describes the time interval required to interrupt the fault current. The time interval in these fuses is approximately inversely proportional to the value of the root mean square of the fault current.
It is desirable to selectively coordinate the recloser with the fuses in order to insure that the reclosure control system in fact interrupts temporary fault currents before the fuses to be protected are blown. This is generally performed by approximating the root mean square value of the fault current by sensing its peak value. It must also be recognized that all faults which occur on a power distribution line are not temporary, such as those caused by a branch momentarily falling against a transmission line. Some faults are of a more permanent nature such as those caused by a transmission line falling to the ground. As a consequence, reclosers are built so that they will only trip a limited number of times within a short duration before locking open. If this not done, the reclosure control system generally would have the recloser cycle until failure and many of the fuses to be protected would be destroyed.
At some magnitude of fault current, the reclosure control system needs to immediately open the recloser to protect the line rather than following the time vs. current characteristic curve. At intermediate levels it may be desirable from the power distribution stand-point to allow the fault current to flow for a limited period to allow the fault to burn itself open or blow the fuse. Many reclosure control systems utilize alternate time vs. current characteristic curves which achieve this goal. Typically, a reclosure control system will allow two shots or trip operations to follow a fast time current characteristic and two additional shots along a somewhat slower time current characteristic before locking open or out.
However, there are a number limitations in the currently available electronic reclosure control systems on the market. One limitation pertains to the method the reclosure control systems utilize to calculate current magnitudes. The presently commercially available reclosure control systems typically use a single scale range for each single current magnitude calculation. By utilizing only a single source, either the accuracy of the pickup current is substantially decreased, or the accuracy for large currents is decreased. Pickup current is defined by the IEEE Standard Definitions for Power Switchgear, ANSI/IEEE Std. C37.100-1992, as the minimum input that will cause a device to complete contact operation. Therefore, by utilizing an inaccurate pickup current value, the reclosure control systems have an increased probability of incorrectly tripping on a current slightly below the pickup value. Furthermore, the available control systems have an increased probability of incorrectly failing to trip on a current slightly above the pickup value.
Another limitation pertains to the method in which the commercially available reclosure control systems compute the necessary fourier transforms. In order for reclosure control systems to calculate the magnitude of current which is present on the power line, a fourier transform is used. Present control systems generally utilize a recursive method of computing the fourier transform. The recursive method of computing the fourier transforms in these systems is inherently prone to cumulative errors. The cumulative errors found in the recursive method are directly caused by the control systems' arithmetical truncation errors.
Another limitation in the commercially available reclosure systems is the use of the fourier transforms just to filter out the harmonics. However, the harmonic values on the power line are never calculated for reporting or alarm purposes.
Still a further limitation in the available reclosure systems pertains to the filtering out of the direct current (DC) component. With most reclosure control systems available on the market, the DC component is filtered out along with the harmonics, prior to the calculation of the fault magnitude. Thus, the magnitude of a fault which contains a decaying DC component is inaccurately represented.
Still another limitation in the available reclosure systems pertains to the rate at which samples of data are taken. Instead of obtaining data samples and performing the fourier transforms on the sampled data at a frequency dependent on the timeliness requirement of the data, the currently available reclosure control systems have a maximum sampling rate of 16 times/cycle.
A further limitation pertains to the number of unnecessary hardware components the available reclosure control systems utilize. The available reclosure control systems require a dual ported random access memory ("RAM") between the digital signal processor ("DSP") and the main processor. These control systems employ the RAM in order to effectuate communication between the DSP and the main processor. In addition, these systems require a high speed, external, read only memory (ROM) chip. Other systems run the DSP program from an external ROM. Thus, either a very expensive high speed ROM is required, or wait-states must be implemented in the DSP execution in order to slow the processing. Alternatively, some control systems download the DSP program from an external ROM into the DSP RAM, thereby enabling the program to execute at full speed. In either case, an extra component which is quite expensive must be utilized.
Thus, one of the objects of the present invention is to provide a method to accurately measure the magnitude of a fault, so that the reclosure control system will not trip on a current level slightly below the pickup value, but will properly trip on a current level slightly above the pickup value.
Another object of the present invention is to utilize a method of computing the fourier transforms which will not be prone to cumulative errors cause by arithmetical truncation errors.
Another object of the present invention is to be able to calculate the harmonic values on power line up to the fifteenth harmonic for reporting and alarm purposes.
Another object of the present invention is to include the DC component in the calculation of the current magnitude in order to accurately represent a fault with a decaying DC component.
Another object of the present invention is to be able to obtain data samples and perform fourier transforms on the sampled data at a frequency dependent on the timeliness requirement of the data.
Another object of the present invention is to be able perform all digital signal processing, storage of data, computations, and communications pertaining to the calculation of current and voltage utilizing a DSP processor, without the use of an external RAM or ROM.
A still further object of the present invention is to be able to perform all digital signal processing, storage of data, computations, and communications without substantially utilizing a wait state.